The IPKat isn’t sure about the TTAB’s reasoning. If a mark has a second meaning, there’s no reason why it can’t remain distinctive (take APPLE for example), though certain uses may not lead to the use of the word being associated with the trade mark owner. What this does go to show though is that trade mark law is firmly in the hands of consumer perception. The fact that consumers have taken the SPAM mark for themselves and granted it a different meaning is decisive, however unfair that may seem to Hormel, who invented the word.
Read the full TTAB decision here.